1. Field of the Invention
The present invention relates to a method and device for discriminating defects of detecting elements constituting a solid-state detector, and more specifically, to a defect discriminating method and a defect discriminating device for discriminating the defective elements among a number of detecting elements constituting a solid-state image sensor, such as a CCD image sensor which detects visible light and outputs an image signal, and a radiation solid-state detector or the like which detects radiation and outputs an image signal.
2. Description of the Prior Art
Up to now, solid-state image sensors such as a CCD image sensor which detects visible light and outputs an image signal have been widely used in such applications as video cameras and digital still cameras. This solid-state image sensor comprises a number of photoelectric transducers arranged in the form of a matrix (for color applications, a color filter is further overlaid upon each photoelectric transducer), outputting an image signal carrying visible image information as two-dimensional matrix information.
Nowadays, in the field of radiation photographing for medical diagnosis, a variety of radiation solid-state detectors which detect radiation and output an image signal (mainly consisting of semiconductors) have been proposed and put to practical use. As a typical one of the various types of radiation solid-state detectors proposed, the radiation solid-state detector of photoelectric conversion type, which reads out the stored charges (also called the xe2x80x9clatent image chargesxe2x80x9d) carrying image information by means of thin film transistors (TFTs) (Japanese Unexamined Patent Publication No. 59 (1984)-211263, Japanese Unexamined Patent Publication No. 2 (1990)-164067, PCT International Publication No. WO92/06501, SPIE Vol. 1443 Medical Imaging V; Image Physics (1991), p. 108-119, etc.), the radiation solid-state detector of direct conversion type (MATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORS, Lawrence Berkeley Laboratory. University of California, Berkeley. Calif. 94720 Xerox Parc. Palo Alto. CA 94304, Metal/Amorphous Silicon Multilayer Radiation Detectors, IEE TRANSACTIONS ON NUCLEAR SCIENCE. VOL. 36. NO. 2. APRIL 1989, Japanese Unexamined Patent Application No. 1 (1989)-216290, etc.), and the radiation solid-state detector of improved direct conversion type, a mode of direct conversion type, (also called xe2x80x9clight reading typexe2x80x9d), in which the reading light is projected for scanning and reading out the latent image charges, are available.
Here, with the radiation solid-state detector of improved direct conversion type, a mode of direct conversion type, i.e., a mode in which the electromagnetic radiation (for example, visible light) for reading is projected for scanning and reading out, which has been proposed in Japanese Patent Application No. 10 (1998)-232824 by the present applicant, a first conductor layer having a permeability to radiation for recording, a photoconductive. layer for recording which exhibits a photoconductive phenomenon (exactly a radiation-conductive one), when irradiated with the radiation for recording which has penetrated through the first photoconductive layer, a charge transporting layer which acts almost as an insulator for a charge having the same polarity as that of the charges provided in the first conductor layer, while acting roughly as a conductor for a charge having a polarity opposite to that of the charges, a photoconductive layer for reading which exhibits a photoconductive phenomenon (exactly an electromagnetic radiation-conductive phenomenon), when irradiated with radiation for reading, and a second conductor layer having a permeability to electoromagnetic radiation for reading, are stacked together in this order, and the latent image charges carrying image information are stored on the boundary surface between the recording photoconductive layer and the charge transporting layer. The first conductor layer and the second conductor layer each act as an electrode. With this mode, the detecting element mainly consists of a photoconductive layer for recording, a charge transporting layer, and a photoconductive layer for reading.
With this improved direct conversion type of radiation solid-state detector, as modes in which the electrostatic latent image carried by the latent image charges are read out, the mode in which the second conductor layer (hereafter called the xe2x80x9creading electrodexe2x80x9d) is formed in a flat sheet, and this reading electrode is scanned with a spot-like beam of a laser or other type of reading light to detect the latent image charges, and the mode in which the reading electrode is provided as stripy electrodes, and the line light sources extending in the direction approximately perpendicular to the longitudinal direction of the stripy electrodes are scanned along the longitudinal direction of the stripy electrodes to detect the latent image charges, are available. With the radiation solid-state detector, whichever reading mode it adopts, it provides a two-dimensional radiation solid-state detector in which a plurality of detecting elements, each of which corresponds to a pixel, are arranged practically in the form of a matrix. The word xe2x80x9cpracticallyxe2x80x9d is used in the above statement because it cannot be said that, with the detector itself, the individual detecting elements are arranged in the form of amatrix. However, in the process in which an image signal obtained by reading out the latent charges is processed, a sampling point corresponds to a pixel, for example. With a radiation solid-state detector in which the reading electrode is provided as stripy electrodes, the stripy electrodes define the pixel points for the direction of arrangement.
With any one of the above-mentioned various types of radiation solid-state detectors, the solid-state detecting elements are arranged in the form of a matrix, and an image signal carrying a radiation image is output as two-dimensional matrix information.
Hereinbelow, a solid-state image sensor which detects visible light and outputs an image signal carrying visible image information, and a radiation solid-state detector which detects radiation and outputs an image signal carrying radiation image information are collectively referred to as xe2x80x9csolid-state detectorsxe2x80x9d. This solid-state detector may not only be two-dimensional, but also one-dimensional.
A variety of elements, such as the photoelectric transducer constituting a solid-state image sensor, and the solid-state detecting elements constituting a radiation solid-state detector (described later) are collectively called xe2x80x9cdetecting elementsxe2x80x9d.
Incidentally, with the above-stated solid-state detector, adherence of dirt during manufacturing, occurrence of scratches in service, etc. may cause a so-called pixel defect, in other words, a phenomenon in which a detection signal is not normally outputted from a specific detecting element of the detecting elements constituting the solid-state detector.
As stated above, this pixel defect is caused by dirt, etc., and is so fine that it is extremely difficult to see by visual inspection. Therefore, a variety of methods which use the digital image processing technology to carry out automatic discrimination of the image defect have been proposed (for example, Japanese Unexamined Patent Publication No. 10 (1998)-133309).
With the defect discriminating method as given in this citation, a flat field image is read out while the recording member (equivalent to the solid-state detector as mentioned in the present specification) is exposed to flat field light, a smooth background image is generated, by removing a value corresponding to it from the pixel value of the flat field image, a residual image is obtained (equivalent to passing through a low-pass filter), and when comparison of the threshold value defined by the dispersion of the noise distribution with the absolute value of the residual image indicates that the absolute value of the residual image is greater than the threshold value, the pixel is discriminated as defective.
However, the above defect discriminating method compares the threshold value (3"sgr" to 5"sgr") defined by the dispersion of the noise distribution with the absolute value of the residual image, and therefore a problem where a normal pixel, if beyond the threshold value (3"sgr" to 5"sgr"), is discriminated as a defective one arises. For example, when the residual image is in the range of variation rather than due to dirt or the like, the pixel is discriminated as a defective pixel regardless of the fact that it is normal. In addition, a problem arises where, because complicated arithmetic processing is required to obtain the residual image, the process is cumbersome.
The first defect discriminating method according to the present invention is a defect discriminating method which discriminates a defective element of a solid-state detector with which a number of detecting elements to detect visible light or radiation are arranged, in which a histogram of each detection signal outputted from the detecting elements in the dark state and/or a histogram of each detection signal outputted from the detecting elements in the bright state in which a definite quantity of light or radiation is projected on to the detecting elements without being passed through a subject are/is acquired, a representative value of the detection signals outputted from said detecting elements which are considered to be normal is determined, the value of the detecting signal at which the frequency in said histogram first comes down to or below a specified frequency, starting from the frequency of said representative value, is taken as the defect discrimination value, and a detecting element among the detecting elements which outputs a detection signal beyond the defect discrimination value is discriminated as a defective element.
The xe2x80x9cdark statexe2x80x9d includes either the state in which visible light or radiation is not projected at all or the state in which a definite and extremely small quantity of visible light or radiation is projected on each detecting element without being passed through a subject. Needless to say, the definite quantity of light in the bright state is greater than that in the dark state.
The xe2x80x9cdetecting elements in the dark statexe2x80x9d means a detector with which radiation image information is recorded in the dark state, i.e., the detecting elements constituting a detector with which radiation image information is practically not recorded, and the xe2x80x9cdetecting elements in the bright statexe2x80x9d means a detector with which radiation image information is recorded in the bright state, i.e., the detecting elements constituting a detector with which a piece of radiation image information is recorded. Needless to say, both detectors mentioned above are the same detector.
In the first defect discriminating method, it is desirable to determine the defect discrimination value which takes the variation in sensitivity of the detecting elements into consideration by determining the xe2x80x9cspecified frequencyxe2x80x9d according to an index which indicates the sensitivity of said detecting elements. Here, xe2x80x9cthe variation in sensitivity of the detecting elementsxe2x80x9d means variation in the output level of the detecting elements caused by gain variation or offset in the conversion gain of the detecting elements or in the output gain of the output amplifier for reading out electric charges. The xe2x80x9cindex indicating the sensitivity of the detecting elementsxe2x80x9d is an index or character which is capable of indicating said xe2x80x9cvariation in sensitivity of the detecting elementsxe2x80x9d. It is, for example, a dispersion of the values obtained by subtracting the detected signal value in the dark state from the detected signal value in the bright state of each detecting element, or more simply, can be a dispersion value of the detected signals in the bright state. It should be noted that the process of obtaining the dispersion value may be implemented only at the time of determining said xe2x80x9cspecified frequencyxe2x80x9d, and may not be carried out every time the defect is discriminated.
In the first defect discriminating method according to the present invention, it is desirable to use the value of the detection signal providing the maximum frequency (including the vicinity thereof) of the histogram or the average value of the detection signals as the representative value of the detection signals outputted from said detecting elements which are considered to be normal. (It is the same for the below-described second defect discriminating method.)
In this case, xe2x80x9cThe value of a detection signal at which the frequency in the histogram first comes down to or below a specified frequency, starting from the frequency of the representative valuexe2x80x9d means at least either of the value at which the frequency first comes down to or below the predetermined specified frequency when the histogram frequency is viewed in the direction in which the value of the detection signal is decreased from the value of the detection signal which provides the frequency in the histogram corresponding to said representative value, and the value at which the frequency first comes down to or below the predetermined specified frequency when the histogram frequency is viewed in the direction in which the value of the detection signal is increased from the value of the detection signal which provides the maximum frequency in the histogram. In this case, it is preferable that the xe2x80x9cspecified frequencyxe2x80x9d be set in consideration of the variation in sensitivity of the detecting elements to provide defect discrimination values for which the variation in sensitivity is taken into account. Hereinbelow, the former (latter) value in the above statement is called the lower (upper) defect discrimination value in the first defect discrimination method.
xe2x80x9cA detection signal beyond the defect discrimination valuexe2x80x9d means a detection signal under the lower defect discrimination value when the lower defect discrimination value is taken as the defect discrimination value, and a detection signal over the upper defect discrimination value when the upper defect discrimination value is taken as the defect discrimination value.
It is preferable that both lower and upper defect discrimination values be used as the defect discrimination values, and in this case, xe2x80x9ca detection signal beyond the defect discrimination valuexe2x80x9d refers, of course, to either a detection signal under the lower defect discrimination value or a detection signal over the upper defect discrimination value.
The second defect discriminating method according to the present invention is a defect discriminating method which discriminates a defective element of a solid-state detector with which a number of detecting elements to detect visible light or radiation are arranged, in which a histogram of each detection signal outputted from the detecting elements in the dark state and/or a histogram of each detection signal outputted from the detecting elements in the bright state in which a definite quantity of light or radiation is projected on the detecting elements without being passed through a subject are/is acquired, a value of the detection signals outputted from said detecting elements which are considered to be normal is determined, a value obtained by multiplying a specified factor by or adding or subtracting a specified value to/from the representative value is taken as the defect discrimination value, and a detecting element among the detecting elements which outputs a detection signal beyond the defect discrimination value is discriminated as a defective element.
Here, xe2x80x9ca specified factorxe2x80x9d is a value over 1 or under 1.
For this second defect discriminating method, it is preferable that the xe2x80x9cspecified factorxe2x80x9d or xe2x80x9cspecified valuexe2x80x9d be set according to the index indicating the sensitivity of the detecting elements to provide defect discrimination values for which the variation in sensitivity of the detecting elements is taken into account. Hereinbelow, the defect discrimination value smaller (greater) than the value of the detection signal providing the maximum frequency is called the lower (upper) defect discrimination value in the second defect discrimination method.
Also with this second defect discriminating method, xe2x80x9ca detection signal beyond the defect discrimination valuexe2x80x9d means a detection signal under the lower defect discrimination value when the lower defect discrimination value is taken as the defect discrimination value, and a detection signal over the upper defect discrimination value when the upper defect discrimination value is taken as the defect discrimination value. Further, it is preferable that both lower and upper defect discrimination values be used as the defect discrimination values, and in this case, xe2x80x9ca detection signal beyond the defect discrimination valuexe2x80x9d refers, of course, to either a detection signal under the lower defect discrimination value or a detection signal over the upper defect discrimination value. The first defect discriminating device according to the present invention is a device which realizes the first defect hi discriminating method, i.e., a defect discriminating device which discriminates a defective element of a solid-state detector with which a number of detecting elements to detect visible light or radiation are arranged, comprising: histogram acquisition means for acquiring a histogram of each detection signal outputted from the detecting elements in the dark state and/or a histogram of each detection signal outputted from the detecting elements in the bright state in which a definite quantity of light or radiation is projected on the detecting elements without being passed through a subject, defect discrimination value determination means for determining a representative value of the detection signals outputted from said detecting elements which are considered to be normal, and taking, as the defect discrimination value, the value of a detection signal among said detection signals at which the frequency in said histogram first comes down to or below a specified frequency, starting from the frequency of said representative value, and discriminator means for discriminating, as a defective element, a detecting element among the detecting elements which outputs a detection signal beyond the defect discrimination value.
The defect discrimination value determination means of the first defect discriminating device is preferred to determine the specified value according to an index indicating the sensitivity of the detecting elements.
The second defect discriminating device according to the present invention is a device which realizes the second defect discriminating method, i.e., a defect discriminating device which discriminates a defective element of a solid-state detector with which a number of detecting elements to detect visible light or radiation are arranged, comprising: histogram acquisition means for acquiring a histogram of each detection signal outputted from the detecting elements in the dark state and/or a histogram of each detection signal outputted from the detecting elements in the bright state in which a definite quantity of light or radiation is projected onto the detecting elements without being passed through a subject, defect discrimination value determination means for determining a representative value of the detection signals outputted from said detecting elements which are considered to be normal, and taking, as the defect discrimination value, a value obtained by multiplying a specified factor by or adding or subtracting a specified value to/from the value of said detection signal at which the maximum frequency is provided, and discriminator means for discriminating, as a defective element, a detecting element among the detecting elements which outputs a detection signal beyond the defect discrimination value.
For this second defect discriminating device, it is preferable that the xe2x80x9cspecified factorxe2x80x9d or xe2x80x9cspecified valuexe2x80x9d be set in consideration of an index indicating the sensitivity of the detecting elements.
In the first and second defect discriminating devices, said defect discrimination value determination means is preferred to use the value of the detection signal providing the maximum frequency of said histogram or the average value of the detection signals as said representative value of the detection signals outputted from said detecting elements which are considered to be normal.
With the first defect discriminating method and device according to the present invention, a histogram of each detection signal in the dark state and/or that of each detection signal in the bright state are/is acquired, a representative value of the detection signals outputted from said detecting elements which are considered to be normal (for example, a value of the detection signal providing the maximum frequency, or the average value of the detection signals) is determined, and the value of a detection signal among said detection signals at which the frequency in said histogram first comes down to or below a specified frequency, starting from the frequency of said representative value, is taken as the defect discrimination value, and any detecting element outputting a detection signal beyond this defect discrimination value is discriminated as a defective element. Therefore, by executing simple arithmetic processing to acquire a histogram, a defect can be discriminated, and thus the first method and device are well suited for automatic discrimination of defects. In addition, if the detection signal his related to the pixel position in processing, the pixel position of a defective element can be identified.
In addition, the discrimination is made by acquiring a histogram, so it is therefore not affected by the quantity of light or other factor, which means that if the quantity of light or other factor is varied in the bright state, the defect can be accurately discriminated.
In addition, if the specified frequency is set at zero, only the abnormal values due to the defective elements which are outside of the normal distribution can be exactly extracted regardless of the variation in sensitivity of the detecting elements, and the normal pixels will not be discriminated as defective pixels.
In addition, if the specified frequency is set according to the index indicating the sensitivity of the detecting elements, the defect can be discriminated in consideration of the variation in sensitivity of the detecting elements, and for example, the elements having an abnormal sensitivity can be discriminated as defective elements.
On the other hand, with the second defect discriminating method and device according to the present invention, a histogram of each detection signal in the dark state and/or that of each detection signal in the bright state are/is acquired, representative value of the detection signals outputted from said detecting elements which are considered to be normal is determined, the value obtained by multiplying a specified factor by or adding or subtracting a specified value to/from said representative value is taken as the defect discrimination value, and any detecting element outputting a detection signal beyond this defect discrimination value is discriminated as a defective element. Therefore, as is the case with the first method and device, by executing simple arithmetic processing, a defect can be discriminated, which makes the second method and device well suited for automatic discrimination of defects, and allows the pixel position of a defective element to be identified.
In addition, as is the case with the first defect discriminating method and device, the discrimination is made by acquiring a histogram, and so it is not affected by the quantity of light or other factor, which means that if the quantity of light or other factor is varied in the bright state, the defect can be accurately discriminated.
In addition, if the specified factor or value is set with the index indicating the sensitivity of the detecting elements, the defect can be discriminated in consideration of the variation in sensitivity of the detecting elements as in the first defect discriminating method and device, and the elements having an abnormal sensitivity can be discriminated as defective elements.
The above-stated methods and devices are applicable not only to the above-stated three modes of radiation solid-state detector, but also to any solid-state detector which mainly consists of semiconductors, and is configured by arranging a number of elements which detect visible light or radiation. For example, they are also applicable to other modes of radiation solid-state detector which detect radiation and output an image signal, solid-state image sensors such as a CCD image sensor which detects visible light and outputs an image signal, etc. This solid-state detector may be either one-dimensional or two-dimensional.
The purpose of the present invention is to offer a defective element discriminating method and device which can conveniently and properly discriminate defects of a detecting element.